Cold Cathode Fluorescent Lamps (CCFL) are used as white-light sources to backlight liquid crystal displays (LCDs). CCFLs are sealed glass tubes filled with inert gases. When a high voltage is placed across the tube, the gases ionize creating ultraviolet (UV) light. The UV light, in turn, excites an inner coating of phosphor, creating visible light. An unusual characteristic of CCFLs is that they exhibit negative impedance. This means that the CCFL voltage drops as current increases. Negative impedance can vary between individual CCFLs, causing the CCFLs to have different currents at any particular voltage level. In multiple-CCFL applications, therefore, the most uniform CCFL performance is achieved by providing individual transformers and current control for each CCFL to ensure a regulated current load. This is a costly and complex solution.
In addition to negative impedance, CCFLs also require high voltages for operation. These voltages typically range from between 200 volts and 2000 volts. At start-up, a CCFL requires substantially higher voltages (20% to 100% higher), referred to as strike voltage, to initiate the lamp plasma. CCFL efficiency is also affected by the current waveform driving the CCFL. Although sinusoidal waveforms provide the greatest efficiency, new driving methods have been developed such that it is possible to drive CCFLs at low frequency with either sine or square waveforms (e.g., between 100 Hz and 1 kHz). Disadvantageously, at these frequencies, it is not practical to use transformers for current dividing due to physical size requirements.
What is desired is a solid state circuit and method of splitting currents, without using a pre-assigned channel for current sensing driving slave channels as current mirrors. The present invention overcomes the disadvantages of a fixed master-slave arrangement wherein it is not known which lamp will have the lowest or highest impedance at any given time.